1. Technical Field
An aspect of the present disclosure relates to an electronic device, and more particularly, to a semiconductor memory device and an operating method thereof.
2. Description of the Related Art
Semiconductor devices, particularly, semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices.
A non-volatile memory device performs a read/write operation at a relatively low speed, but retains stored data even when a power supply is cut off. Accordingly, the non-volatile memory device is used to store data to be retained regardless of a power supply. Examples of non-volatile memory devices are a read-only memory (ROM), a mask ROM (MROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a phase-change random access memory (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), a ferroelectric RAM (FRAM), and the like. Flash memories are classified into NOR type flash memories and NAND type flash memories.
The flash memories have an advantage over RAM in which data is freely programmed and erased. Flash memories also have an advantage over ROM in which stored data is retained even when a power supply is cut off. The flash memories are widely used as a storage medium of portable electronic devices such as a digital camera, a personal digital assistant (PDA), and an MP3 player.
Flash memory may be classified into a two-dimensional semiconductor device and a three-dimensional semiconductor device each having a string vertically formed on a semiconductor substrate.
The three-dimensional semiconductor device is a memory device devised in order to overcome degree of integration limits found in two-dimensional semiconductor devices and includes a plurality of memory strings vertically formed on a semiconductor substrate. The memory strings include a drain select transistor, memory cells, and a source select transistor, connected in series between a bit line and a source line.